Because of their controlled nature, the presence of independent replicates,
and their known management history, long-term field experiments are key to
the understanding of factors controlling soil carbon. Together with isotope
measurements, they provide profound insight into soil carbon dynamics. For
soil radiocarbon, an important tracer for understanding these dynamics,
experimental variability across replicates is usually not accounted for;
hence, a relevant source of uncertainty for quantifying turnover rates is
missing. Here, for the first time, radiocarbon measurements of five
independent field replicates, and for different layers, of soil from the
66-year-old controlled field experiment ZOFE in Zurich, Switzerland, are
used to address this issue. 14C variability was the same across three
different treatments and for three different soil layers between the surface and
90 cm depths. On average, experimental variability in 14C content was
12 times the analytical error but still, on a relative basis, smaller than
variability in soil carbon concentration. Despite a relative homogeneous
variability across the field and along the soil profile, the curved nature
of the relationship between radiocarbon content and modelled carbon mean
residence time implies that the absolute error of calculated soil carbon
turnover time increases with soil depth. In our field experiment findings on
topsoil carbon turnover variability would, if applied to subsoil, tend to
underweight turnover variability even if experimental variability in the
subsoil isotope concentration is the same. Together, experimental
variability in radiocarbon is an important component in an overall
uncertainty assessment of soil carbon turnover.